Various known techniques exist for embedding information into a printed work, for example, a legal document or currency. The embedded information may be visual information, which can be embedded into the document by treating the document. For example, micro printing produces an image that is too small to be discerned by the naked eye. The size, for example, of the image may be one hundredth of a normal type size. While features of this size can be easily printed, they are difficult to copy or scan. The printed feature is thus covert, and may be viewed using a magnifying glass to confirm the fact that the printed item is an original.
Another technique that is used to authenticate documents is called the void pantograph. Printed dots of different sizes are utilized to create an effect. Smaller dots visible to the human eye dominate the appearance of the form. However the smaller dots are lost when a facsimile is produced in copying or scanning. Then, larger, less frequently printed dots become visible in the copy, and spell out the word “void”.